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What happens in the stroma of a chloroplast?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — Photosynthesis Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

The stroma is a colorless, enzyme-rich matrix within the chloroplast that serves as the site for the "dark reactions" (or carbon-assimilating reactions) of photosynthesis.12

In the stroma, the plant utilizes the ATP and NADPH generated during the light reactions to convert carbon dioxide into carbohydrates. This process primarily occurs through the Calvin-Benson cycle, which consists of three main stages:134

Carboxylation: The enzyme RuBisCO attaches carbon dioxide to a five-carbon compound (RuBP), creating an unstable molecule that splits into three-carbon compounds.42
Reduction: Using energy from ATP and electrons from NADPH, these three-carbon compounds are converted into high-energy sugar molecules, such as glyceraldehyde-3-phosphate.34
Regeneration: A portion of the resulting compounds is used to regenerate RuBP, allowing the cycle to continue.42

The stroma also contains starch granules, which serve as storage for the products of these reactions.1